Self-adjusting braking apparatus

ABSTRACT

A self-adjusting braking apparatus, particularly for railmounted vehicles, comprises two brakeshoes which are movable against opposite sides of a disk by two levers each of which has one end pivotally connected to a shoe. The other end of one of the levers is pivoted to the frame of the vehicle and the other end of the other lever is movable with reference to the other end of the one lever by a braking cylinder. A coupling device of infinitely variable length connects intermediate portions of the two levers so that the levers pivot with reference to the coupling device while the other end of the other lever is subjected to the action of the braking force whereby the shoes move toward the respective sides of the disk. The length of the coupling device is automatically reduced in response to engagement of a projection of the coupling device with an abutment provided on a third lever one end of which is connected to the one end of the other lever and the other end of which is pivoted to the frame. Shortening of the coupling device compensates for wear on linings of the brakeshoes.

United States Patent Otto et al. 51 Feb. 29, 1972 [54] SELF-ADJUSTINGBRAKING 3,249,l35Il56 Billeterllj..IQ...L. .1LI.IIIl l 837T9? YAPPARATUS 1 Primary Examiner--Duane A. Reger [72] lnventors: AlfredOtto, Remscheid; Horst Klassen, St. A -Mi h l S, s ik Sebastian, both ofGermany; Daniel Steiner, Schaffhausen, Switzerland [57] ABSTRACT [73]Assignees: Bergische Stahl-lndustrie, Remscheid, A self-adjustingbraking apparatus, particularly for rail- Germany; Schweizerischelndustriemounted vehicles, comprises two brakeshoes which are mova-Gesellschafl, Neuhausen am Rh i f ll, ble against opposite sides of adisk by two levers each of which S i l d has one end pivotally connectedto a shoe. The other end of one of the levers is pivoted to the frame ofthe vehicle and the [22] Wed: June 1969 other end of the other lever ismovable with reference to the [211 App]. No; 831,861 other end of theone lever by a braking cylinder. A coupling device of infinitelyvariable length connects intermediate portions of the two levers so thatthe levers pivot with reference to [30] Foreign Application PriorityData the coupling device while the other end of the other lever issub'ected to the action of the brakin force whereb the shoes June 11,1968 Germany ..P 17 50 849.5 l toward the respective Sides oftgh diskThe leigth ofthe coupling device is automatically reduced in response toen- [52] US. Cl. ..l88/ 196 D, 188/59, 188/202 gagemem of a Projection fthe coupling device with an ablm [51] Int. Cl ..F16d 65/66 mem providedon a third ever one end f which is connected of Search 7 1 1 .9, to theone end of the other lever and the other end of which is 196 263 PLpivoted to the frame. Shortening of the coupling device compensates forwear on linings of the brakeshoes. [56] References Cited 19 Claims, 3Drawing Figures UNITED STATES PATENTS A Mk W 3,141,861 9/1964 Hodkinson.L ..l88/l96 D x i m w PMENIEDrzaze I972 VIII,

In venfors. nor/IE9 arTzJ Hour tmsmw [40154 $75106 SELF-ADJUSTINGBRAKING APPARATUS BACKGROUND OF THE INVENTION The present inventionrelates to braking apparatus in general, and more particularly toimprovements in self-adjusting braking apparatus (i.e., apparatus whichautomatically compensate for wear on their friction generating parts)which are especially suited for use in rail-mounted vehicles.

Certain conventional self-adjusting braking apparatus comprise a brakingcylinder whose piston rod is caused to change its effective length inorder to compensate for wear on the linings of brakeshoes. This insuresthat the distance which the brakeshoes must cover to move their liningsinto engagement with the disk is constant. A drawback of such brakingapparatus is that they comprise a large number of complicated parts andthat the length of the cylinder is excessive, i.e., it is dimcult toprovide in a vehicle enough room to accommodate such self-adjustingapparatus. Certain more recent proposals include the provision of anadjustable bridge which is used in the linkage for the brakeshoes;however, such bridge is adjustable stepwise which is unsatisfactorybecause the clearance between the brakeshoes and the disk prior toapplication of the braking force is not constant.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a braking apparatus which adjusts itself automatically inresponse to wear on the linings of its brakeshoes and which is capableof effecting an infinite number of adjustments.

Another object of the invention is to provide a reliable selfadjustingbraking apparatus wherein the adjustment is effected at a point which isremote from the brake cylinder and which can insure that the clearancebetween the linings of brakeshoes and the disk remains constant when theapparatus is idle, regardless of the extent of wear on the linings.

A further object of the invention is to provide a novel infinitelyadjustable and readily resettable coupling device for use in the aboveoutlined braking apparatus and to design the coupling device in such away that it can be installed in existing braking apparatus bynecessitating only minor alterations in such apparatus.

An additional object of the invention is to provide a compactself-adjusting braking apparatus which comprises a relatively smallnumber of parts and which is also adjustable by hand if the person incharge wishes to change the clearances between the linings of brakeshoesand the disk.

An ancillary object of the invention is to provide a braking apparatuswhich is particularly suited for use in rail-mounted vehicles and whichcan be reset in a time-saving operation when the one and/or the otherlining is to be replaced with a fresh lining.

The improved self-adjusting braking apparatus comprises a rotary member(e.g., a disk or the wheel of a rail-mounted vehicle), a pair ofbrakeshoes movable toward and away from engagement with opposite sidesof the rotary member and having linings which can engage liners at therespective sides of the rotary member, a first lever having one endpreferably articulately connected to one of the shoes and another endpivotable about a fixed axis which is defined by the frame of thevehicle, a second lever having one end preferably articulately connectedwith the other shoe and another end which moves with reference to theother end of the first lever in response to application of a brakingforce which can be furnished by a hydraulic or pneumatic cylinder, anadjustable coupling device or bridge of infinitely variable length whichis articulately connected to intennediate portions of the two levers sothat the levers can pivot with reference to two relatively movablecoupling members of the coupling device in response to application ofthe braking force to thereby move the respective shoes toward engagementwith the rotary member, and adjusting means which is operative to reducethe length of the coupling device (i.e., the distance between theintermediate portions of the levers) when the distance between the otherends of the two levers reaches a predetermined value during applicationof the braking force. Such displacement of the other end of the secondlever relative to the other end of the one lever beyond (above or below)the predetermined distance is indicative of wear on one or both liningsand a shortening of the coupling device compensates for such wear.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theimproved braking apparatus itself, however, both as to its constructionand its mode of operation, together with additional features andadvantages thereof, will be best understood upon perusal of thefollowing detailed description of certain specific embodiments withreference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic plan view of aself-adjusting braking apparatus which embodies one form of theinvention;

FIG. 2 is a plan view of a slightly modified braking apparatus; and

FIG. 3 is an enlarged axial sectional view of the novel infinitelyadjustable coupling device as seen in the direction of arrows from theline IIIIII of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates aself-adjusting braking apparatus which includes a rotary member hereshown as a disk 11 which is mounted on the axle of a vehicle, forexample, on the axle of a railroad car. The disk 11 can constitute thewheel of a vehicle. The sides of the disk 11 are provided with tworing-shaped liners 11a, 11b which are respectively adjacent tobrakeshoes 12, 13. These brakeshoes are preferably suspended on theframe of the vehicle. The improved linkage which can move the linings ofbrakeshoes 12, 13 toward and away from frictional engagement with theliners 11a, 11b comprises a first lever 14 one end of which is pivotallyconnected to the frame of the vehicle (as at 15) to turn about a fixedaxis, and the other end of which is pivotally connected to the brakeshoe12. A second lever 16 of the linkage is articulately connected with thebrakeshoe 13 at one end and has a second end remote from the brakeshoe13 and adapted to be subjected to the action of a braking force P actingin a direction to pivot the lever 16 clockwise, as viewed in FIG. 1.Such braking force can be furnished by a hydraulic or pneumaticcylinder. The levers 14, 16 have intermediate portions which arearticulately connected to a coupling device or bridge 17 of infinitelyadjustable length. A third lever 18 which forms part of an adjustingdevice for the bridge 17 is articulately connected to the brakeshoe 13and lever 16 at one end and to the frame at the other end (as shown at19) so that it can turn about a fixed axis. The bridge 17 is providedwith a projection 20 which can engage an anvil or abutment 21 on anintermediate portion of the third lever 18. When the braking apparatusis idle, there exists at least some clearance or gap between theprojection 20 and anvil 21. The width of this gap determines the play ofthe braking apparatus. The projection 20 is biased outwardly, i.e.,toward the anvil 21, in a manner to be described in greater detail inconnection with FIG. 3 and serves as a means for triggering a change ineffective length of the bridge 17.

The operation If the lining of the brakeshoe 12 has been subjected toextensive wear, the application of braking force I in the directionindicated by arrows and the ensuing pivotal movement of lever 16 in aclockwise direction, as viewed in FIG. 1, results in movement of thebrakeshoe 13 toward and into engagement with the liner 11b of the disk11. The lever 16 thereupon continues to pivot in a clockwise directionwhereby it entrains the lever 14 by way of the bridge 17 until the wornlining of the brakeshoe 12 reaches and frictionally engages the liner110. Since the lining of the brakeshoe 12 is worn, the projection 20 ofthe bridge "reaches the anvil 21 on the third lever 18. Further pivotalmovement of the lever 16 in a clockwise direction results in adepression of the projection 20 and thereby causes a reduction ineffective length of the bridge 17. All this takes place before thelining of the brakeshoe 12 engages the liner 11a. Depression of theprojec-' tion 20 begins when the aforementioned clearance or gap betweenthe projection and anvil 21 is reduced to zero and while the lever 16continues to pivot in a clockwise direction, i.e., while the lining ofthe brakeshoe 12 moves toward the liner 1 1a and the distance betweenthe left-hand ends of levers 14, 16 increases beyond a predeterminedvalue. Such reduction in effective length of the bridge 17 insures thatthe lever 16 must turn through a smaller angle during the nextapplication of braking shoes 12 and that such angular displacement ofthe lever 16 suffices to bring about satisfactory frictional engagementbetween the disk 11 and the linings of both brakeshoes. Thus, ashortening of the bridge 17 results in automatic adjustment of thebraking apparatus to compensate for wear on the lining of the brakeshoe12 or 13. During the next-following application of brakeshoes 12, 13,the linings of both shoes engage the corresponding liners 11a, 11b assoon as the gap between the projection 20 and anvil 21 is reduced tozero.

An advantage of the improved braking apparatus is that its efficiencyexceeds the efficiency of conventional braking apparatus with adjustablepiston rods or with stepwise adjustable coupling devices. This is due tothe fact that the brakeshoes 12, 13 receive only such motion which isneeded to move them intoengagement with the liners of the disk 11. Themovement needed for changing the length of the bridge 17 is not sharedby the brakeshoes. This insures that the levers 14, 16 need not be movedthrough angles of excessive magnitude. The improved infinitelyadjustable bridge 17 (one embodiment of which will be described ingreater detail in connection with FIG. 3) can be installed in existingbraking apparatus with minimal alterations in the construction and modeof operation of such apparatus.

The braking apparatus of FIG. 2 is practically identical with theapparatus of FIG. 1, with the single main exception that the lever 18'is bent or curved so as to permit unobstructed application of brakingforce P to the left-hand end of the lever 16. The force P is preferablytransmitted by a linkage which receives motion from a brake cylinder,not shown. The configuration of the lever 16 can be selected in such away that its left-hand end approaches the left-hand end of the lever 14during application of the braking force P.

FIG. 3 illustrates a portion of the braking apparatus which is shown inFIG. 2. More particularly, FIG. 3 shows the details of the couplingdevice or bridge 17. This bridge comprises a pair of relatively movablecoupling members one of which constitutes a spindle 30 and the other ofwhich constitutes a hollow carrier 31. One end of the spindle 30 isarticulately connected to a pin 14a extending between the two prongs ofthe adjoining bifurcated intermediate portion of the lever 14. The

carrier 31 has a bifurcated end portion which is remote from the pin 14aand whose prongs are coupled, by pins 160, to adjoining prongs of thebifurcated intermediate portion of the lever 16. The carrier 31accommodates the projection which extends through its right-hand openend. The projection 20 can be shifted in a direction to the left, asviewed in FIG. 3, against the opposition of a biasing means here shownas a helical spring 33. The extent to which the projection 20 can moveoutwardly, namely, in a direction to the right, is determined by a stopring 34 which is fitted into an internal groove of the carrier 31. Theinner (left-hand) end portion of the projection 20 is connected with astop ring 35 which surrounds with clearance an externally threadedportion 36 of the spindle 30. A smooth-surfaced tip 37 of the spindleextends into a female portion of the projection 20 so that theprojection serves as a guide for the adjoining part of the spindle. Theexternally threaded portion 36 of the spindle 30 meshes with a nut38a'portion of which is rotatable in a sleevelike bearing 39 extendingthrough the left-hand open end of the carrier 31 and having an exposedportion surrounding a portion of the sleeve 30. The bearing 39 has aflange in the left-hand portion of the carrier 31. The nut 38 isconnected with a helical torsion spring 40 which tends to rotate the nutin a direction to move axially toward the intermediate portion of thelever 14 until a radial shoulder of the nut engages the adjoining radialshoulder on the flange of the bearing 39. The externally threadedportion 36 of the spindle 30 further meshes with a setting nut 41 whichis connected with a second helical torsion spring 42. The latter tendsto rotate the nut 41 to move it axially toward the intermediate portionof the lever 14 until the nut 41 engages the stop ring 35 on theprojection 20. The nuts 38, 41 are provided with entraining pins 43.

An extension or head 44 of the projection 20 can be moved intoengagement with the abutment or anvil here shown as including a screw 46which is rotatable in an internally threaded holder 45 affixed to thelever 18' (not shown in FIG. 3). The bearing 39 and projection 20 areprovided with sealing rings 47 which prevent penetration of dust,moisture or other foreign matter into the interior of the hollow carrier31. The numeral 48 denotes the clearance or gap between the head 44 ofthe projection 20 and the tip of the axially adjustable screw 46.

The coupling device or bridge of FIG. 3 functions as follows:

If the clearance increases due to wear on the lining of the brakeshoe 12or 13, the head 44 of the projection 20 engages the screw 46 duringapplication of the braking force P after the clearance 48 is reduced tozero. The lever 16 continues to move in a direction to the right, asviewed in FIG. 3, whereby the projection penetrates deeper into thecarrier 31 and stresses the spring 33. Such axial movement of theprojection 20 results in movement of the ring 35 away from the nut 41(in a direction to the left) so that the latter receives torque from thespring 42 and moves axially of the externally threaded portion 36 towardthe bearing 39. Rotation and resulting axial movement of the nut 41 isterminated when its lefi-hand end face again engages the stop ring 35.Such axial adjustment of the nut 41 takes place during braking so thatthere is enough time to complete its movement into renewed engagementwith the stop ring 35 on the depressed projection 20.

If the braking force P is thereupon reduced, the head 44 of theprojection 20 remains in engagement with the screw 46 but the carrier 31moves in a direction to the left under the action of the spring 33. Thebearing 39 shares such movement of the carrier 31 relative to thespindle 30, i.e., it moves in a direction toward the intermediateportion of the lever 14 and away from the nut 38. Consequently, the nut38 can rotate with reference to the externally threaded portion 36 underthe action of spring 40 and moves to the left, namely, into renewedengagement with the flange of the bearing 39. During such rotation ofthe nut 38, the carrier 31 cannot move with reference to the projection20 because the latter engages the stop 34. Consequently, the extent ofaxial movement of the nut 38 is a function of the extent of precedingaxial movement of the nut 41. Thus, the reduction in effective length ofthe bridge 17 corresponds to the extent to which the projection 20 isdepressed into the carrier 31 subsequent to elimination of the gap 48.When the application of the braking force P is terminated, the head 44moves away from the screw 46 to reestablish the gap 48. The width ofthis gap can be adjusted by rotating the screw 46 with reference to itsholder 45.

If the linings of the brakeshoes 12, 13 are totally worn away to such anextent that they require replacement, the bearing 39 is rotated by atool which engages its exposed left-hand portion. Rotation of thebearing 39 takes place in a direction counter to that in which the nut38 rotates in order to move toward engagement with the bearing 39. Thepins 43 of the nut 38 entrain the nut 41 and the spindle 30 is movedaxially of the carrier 31 to reestablish the original maximum axiallength of the bridge 17. Upon the first application of brakeshoes withnew linings, the effective length of the bridge 17 isautomaticallyadjusted in such a way that the head 44 t and screw 46 provide a gap 48of desired width. Subsequent changes in effective length of the bridge17 take place automatically in response to progressing wear on thelinings in a manner as described above.

It is clear that the improved braking apparatus is susceptible of manymodifications without departing from the spirit of the invention. Forexample, the screw 46 can be fixedly secured to the lever 18 or 18' ifthe latters end 19 is adjustable with reference to the frame and hencewith reference to the end of the lever 14. This can be achieved byemploying a connector which is an eccentric post or bolt so that itsrotation with reference to the frame effects the same adjustment as therotation of screw 46 with reference to its holder 45. It is alsopossible to change the mounting of the projection 20 in the carrier 31and to change the manner in which the projection 20 cooperates with thelever 18 or 18' or with an anvil on such lever. The spring 33 can bereplaced by a spring which is expanded in response to penetration of theprojection 20 into the carrier 31. Also, the left-hand ends 15, 19 ofthe levers 14, 18 or 14, 18' can be mounted for pivotal movement about acommon axis.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featureswhich fairly constitute essential characteristics of the generic andspecific aspects of our contribution to the art.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

1. Self-adjusting braking apparatus, particularly for use inrail-mounted vehicles, comprising a rotary member; a pair of brakeshoesmovable toward and away from engagement with opposite sides of saidrotary member; a first lever having one end connected to one of theshoes and another end pivotable about a fixed axis; a second leverhaving one end connected to the other shoe and another end arranged tomove with reference to the other end of said first lever in response toapplication of a braking force; an adjustable coupling device ofinfinitely variable length articulately connected with intermediateportions of said levers so that the levers can pivot with reference tothe coupling device in response to application of the braking force tothereby move the respective shoes toward engagement with said rotarymember; and adjusting means operative to reduce the length of saidcoupling device when the distance between the other ends of said leversreaches a predetermined value during application of the braking force.

2. Self-adjusting braking apparatus as defined in claim 1, wherein saidadjusting means comprises a third lever having one end connected to saidother shoe and another end tumable about a predetermined axis, and anabutment provided on said third lever in the path of movement of saidcoupling device during application of braking force to the other end ofsaid second lever.

3. Self-adjusting braking apparatus as defined in claim 2, wherein saidcoupling device comprises carrier means and a projection which isnormally spaced from said abutment and is displaced with reference tosaid carrier means in response to engagement with said abutment duringmovement of the other end of said second lever relative to the other endof said first lever beyond said predetermined distance.

4. Self-adjusting braking apparatus as defined in claim 2, wherein saidthird lever is curved.

5. Self-adjusting braking apparatus as defined in claim 2, wherein saidpredetermined axis for the other end of said third lever is adjustablewith reference to said fixed axis.

6. Self-adjusting braking apparatus as defined in claim 2, wherein theother ends of said first and third levers are turnable about a commonaxis.

7. Self-adjusting braking apparatus as defined in claim 1, wherein theone end of each of said levers is articulately connected with therespective shoe.

8. Self-adjusting braking apparatus as defined in claim 1, wherein saidadjusting means comprises an abutment which is adjustable with referenceto said coupling means.

9. An adjustable coupling device of variable length, particularly forconnecting the levers for brakeshoes in a braking apparatus, forrail-mounted vehicles, comprising a first coupling member constituting ahollow carrier having two ends, a pro jection reciprocably received insaid first coupling member and extending from one end thereof, a secondcoupling member reciprocable with reference to said first couplingmember and said projection, said second coupling member extendingthrough the other end of said first coupling member and comprising anexternally threaded portion in the interior of said first couplingmember, and means for moving one of said coupling members with referenceto the other coupling member to thereby change the combined length ofsaid coupling members in response to movement of said projection withreference to said first coupling member, said means for movingcomprising a pair of nuts meshing with said externally threaded portionand torsion springs for rotating said nuts with reference to saidexternally threaded portion in response to movement of said projectiontoward the other end of said first coupling member.

10. An adjustable coupling device of variable length, particularly forconnecting the levers for brakeshoes in a braking apparatus forrail-mounted vehicles, comprising a first coupling member constituting ahollow carrier having two ends, a projection reciprocably received insaid first coupling member and extending from one end thereof, a secondcoupling member reciprocable with reference to said first couplingmember and said projection, said second coupling member extendingthrough the other end of said first coupling member and comprising anexternally threaded portion in the interior of said first couplingmember, and means for moving one of said coupling members with referenceto the other coupling member to thereby change the combined length ofsaid coupling members in response to movement of said projection withreference to said first coupling member, said means for movingcomprising a pair of nuts meshing with said externally threaded portionand means for rotating said nuts with reference to said externallythreaded portion in response to movement of said projection toward theother end of said carrier, said projection comprising stop means locatedbetween said nuts and arranged to terminate rotary movement of one ofsaid nuts in response to movement of said projection toward the othernut.

11. A coupling device as defined in claim 9, further comprising meansfor biasing said projection away from the other end of said firstcoupling member.

12. A coupling device as defined in claim 10, wherein said stop meanscomprises a ring.

13. A coupling device as defined in claim 9, further comprising anannular bearing mounted in said carrier adjacent to one of said nuts,said bearing having a portion extending through the other end of saidcarrier and surrounding a portion of said second coupling member.

14. A coupling device as defined in claim 9, further comprising meansprovided in said carrier for biasing said projection away from saidother end and means for limiting the extent of movement of saidprojection under the action of said biasing means.

15. A coupling device as defined in claim 2, wherein at least one ofsaid nuts is provided with entraining means for rotating the other nutin response to rotation of said one nut.

16. An adjustable coupling device of variable length, particularly forconnecting the levers for brakeshoes in a braking apparatus forrail-mounted vehicles, comprising a first coupling member constituting ahollow carrier having two ends, a projection reciprocably received insaid first coupling member and extending form one end thereof, a secondcoupling member reciprocable with reference to said first couplingmember and said projection, said second coupling member extendingthrough the other end of said first coupling member and comprising anexternally threaded portion in the interior of said first couplingmember, said projection comprising a female portion and said secondcoupling member further comprising an end portion reciprocably extendinginto said female portion, and means for moving one of said couplingmembers with reference to the other coupling member to thereby changethe combined length of said coupling members in response to movement ofsaid projection with reference to said first coupling member, said meansfor moving comprising a pair of nuts meshing with said externallythreaded portion and means for rotating said nuts with reference to saidexternally threaded portion in response to movement of said projectiontoward the other end of said first coupling member.

17. An adjustable coupling device of variable length, particularly forconnecting the levers for brakeshoes in a braking apparatus forrail-mounted vehicles, comprising a first coupling member constituting ahollow carrier having two ends, a projection reciprocably received insaid first coupling member and extending from one end thereof, a secondcoupling member reciprocable with reference to said first couplingmember and said projection, said second coupling member extendingthrough the other end of said first coupling member and comprising anexternally threaded portion in the interior of said first couplingmembers, means for moving one of said coupling members with reference tothe other coupling member to thereby change the combined length of saidcoupling members in response to movement of said projection withreference to said first coupling member, said means for movingcomprising a pair of nuts meshing with said externally threaded portionand means for rotating said nuts with reference to said externallythreaded portion in response to movement of said projection toward theother end of said first coupling member, an annular bearing mounted insaid first coupling member adjacent to one of said nuts and having aportion extending through said other end of said first coupling memberand surrounding a portion of said second coupling member, and sealingmeans interposed between said projection and said first coupling member.

18. A coupling device as defined in claim 2, wherein said nuts arearranged to move toward the other end of said carrier in response tomovement of said projection in the same direction.

19. A coupling device as defined in claim 2, further comprising meansfor biasing said projection to an outer end position and wherein saidnuts are rotatable in a direction toward said other end of the carrier,the latter comprising stop means engaging one of said nuts upon movementof said carrier and said second coupling member in opposite directionsto permit movement of said one nut awayfrom said stop means whereby theone nut is free to rotate with reference to said externally threadedportion to return into abutment with said stop means.

t i 8! 1i t

1. Self-adjusting braking apparatus, particularly for use inrail-mounted vehicles, comprising a rotary member; a pair of brakeshoesmovable toward and away from engagement with opposite sides of saidrotary member; a first lever having one end connected to one of theshoes and another end pivotable about a fixed axis; a second leverhaving one end connected to the other shoe and another end arranged tomove with reference to the other end of said first lever in response toapplication of a braking force; an adjustable coupling device ofinfinitely variable length articulately connected with intermediateportions of said levers so that the levers can pivot with reference tothe coupling device in response to application of the braking force tothereby move the respective shoes toward engagement with said rotarymember; and adjusting means operative to reduce the length of saidcoupling device when the distance between the other ends of said leversreaches a predetermined value during application of the braking force.2. Self-adjusting braking apparatus as defined in claim 1, wherein saidadjusting means comprises a third lever having one end connected to saidother shoe and another end turnable about a predetermined axis, and anabutment provided on said third lever in the path of movement of saidcoupling device during application of braking force to the other end ofsaid second lever.
 3. Self-adjusting braking apparatus as defined inclaim 2, wherein said coupling device comprises carrier means and aprojection which is normally spaced from said abutment and is displacedwith reference to said carrier means in response to engagement with saidabutment during movement of the other end of said second lever relativeto the other end of said first lever beyond said predetermined distance.4. Self-adjusting braking apparatus as defined in claim 2, wherein saidthird lever is curved.
 5. Self-adjusting braking apparatus as defined inclaim 2, wherein said predetermined axis for the other end of said thirdlever is adjustable with reference to said fixed axis.
 6. Self-adjustingbraking apparatus as defined in claim 2, wherein the other ends of saidfirst and third levers are turnable about a common axis. 7.Self-adjusting braking apparatus as defined in claim 1, wherein the oneend of each of said levers is articulately connected with the respectiveshoe.
 8. Self-adjusting braking apparatus as defined in claim 1, whereinsaid adjusting means comprises an abutment which is adjustable withreference to said coupling means.
 9. An adjustable coupling device ofvariable length, particularly for connecting the levers for brakeshoesin a braking apparatus, for rail-mounted vehicles, comprising a firstcoupling member constituting a hollow carrier having two ends, aprojection reciprocably received in said first coupling member andextending from one end thereof, a second coupling member reciprocablewith reference to said first coupling member and said projection, saidsecond coupling member extending through the other end of said firstcoupling member and comprising an externally threaded portion in theinterior of said first coupling member, and means for moving one of saidcoupling members with reference to the other coupling member to therebychange the combined length of said coupling members in response tomovement of said projection with reference to said first couplingmember, said means for moving comprising a pair of nuts meshing withsaid externally threaded portion and torsion springs for rotating saidnuts with reference to said externally threaded portion in response tomovement of said projection toward the other end of said first couplingmember.
 10. An adjUstable coupling device of variable length,particularly for connecting the levers for brakeshoes in a brakingapparatus for rail-mounted vehicles, comprising a first coupling memberconstituting a hollow carrier having two ends, a projection reciprocablyreceived in said first coupling member and extending from one endthereof, a second coupling member reciprocable with reference to saidfirst coupling member and said projection, said second coupling memberextending through the other end of said first coupling member andcomprising an externally threaded portion in the interior of said firstcoupling member, and means for moving one of said coupling members withreference to the other coupling member to thereby change the combinedlength of said coupling members in response to movement of saidprojection with reference to said first coupling member, said means formoving comprising a pair of nuts meshing with said externally threadedportion and means for rotating said nuts with reference to saidexternally threaded portion in response to movement of said projectiontoward the other end of said carrier, said projection comprising stopmeans located between said nuts and arranged to terminate rotarymovement of one of said nuts in response to movement of said projectiontoward the other nut.
 11. A coupling device as defined in claim 9,further comprising means for biasing said projection away from the otherend of said first coupling member.
 12. A coupling device as defined inclaim 10, wherein said stop means comprises a ring.
 13. A couplingdevice as defined in claim 9, further comprising an annular bearingmounted in said carrier adjacent to one of said nuts, said bearinghaving a portion extending through the other end of said carrier andsurrounding a portion of said second coupling member.
 14. A couplingdevice as defined in claim 9, further comprising means provided in saidcarrier for biasing said projection away from said other end and meansfor limiting the extent of movement of said projection under the actionof said biasing means.
 15. A coupling device as defined in claim 2,wherein at least one of said nuts is provided with entraining means forrotating the other nut in response to rotation of said one nut.
 16. Anadjustable coupling device of variable length, particularly forconnecting the levers for brakeshoes in a braking apparatus forrail-mounted vehicles, comprising a first coupling member constituting ahollow carrier having two ends, a projection reciprocably received insaid first coupling member and extending form one end thereof, a secondcoupling member reciprocable with reference to said first couplingmember and said projection, said second coupling member extendingthrough the other end of said first coupling member and comprising anexternally threaded portion in the interior of said first couplingmember, said projection comprising a female portion and said secondcoupling member further comprising an end portion reciprocably extendinginto said female portion, and means for moving one of said couplingmembers with reference to the other coupling member to thereby changethe combined length of said coupling members in response to movement ofsaid projection with reference to said first coupling member, said meansfor moving comprising a pair of nuts meshing with said externallythreaded portion and means for rotating said nuts with reference to saidexternally threaded portion in response to movement of said projectiontoward the other end of said first coupling member.
 17. An adjustablecoupling device of variable length, particularly for connecting thelevers for brakeshoes in a braking apparatus for rail-mounted vehicles,comprising a first coupling member constituting a hollow carrier havingtwo ends, a projection reciprocably received in said first couplingmember and extending from one end thereof, a second coupling memberreciprocable with reference to said first coupling member and saidprojection, sAid second coupling member extending through the other endof said first coupling member and comprising an externally threadedportion in the interior of said first coupling members, means for movingone of said coupling members with reference to the other coupling memberto thereby change the combined length of said coupling members inresponse to movement of said projection with reference to said firstcoupling member, said means for moving comprising a pair of nuts meshingwith said externally threaded portion and means for rotating said nutswith reference to said externally threaded portion in response tomovement of said projection toward the other end of said first couplingmember, an annular bearing mounted in said first coupling memberadjacent to one of said nuts and having a portion extending through saidother end of said first coupling member and surrounding a portion ofsaid second coupling member, and sealing means interposed between saidprojection and said first coupling member.
 18. A coupling device asdefined in claim 2, wherein said nuts are arranged to move toward theother end of said carrier in response to movement of said projection inthe same direction.
 19. A coupling device as defined in claim 2, furthercomprising means for biasing said projection to an outer end positionand wherein said nuts are rotatable in a direction toward said other endof the carrier, the latter comprising stop means engaging one of saidnuts upon movement of said carrier and said second coupling member inopposite directions to permit movement of said one nut away from saidstop means whereby the one nut is free to rotate with reference to saidexternally threaded portion to return into abutment with said stopmeans.